1. Field of the Invention
The invention relates to a motor control unit and a vehicle that is equipped with the motor control unit.
2. Description of the Related Art
Motor drive control systems in which a motor is driven by an inverter are utilized in various fields. In such systems, an inverter circuit for driving a motor includes switching devices such as an IGBT device, a power MOS device and the like. Since these switching devices can be damaged by high temperature, torque is normally limited when the temperature of the inverter increases.
Japanese Patent Application Publication No. JP-A-9-121595 describes a thermal protection device for a power converter, which is capable of thermally protecting a switching device of an inverter circuit without decreasing torque even if the temperature of the switching device becomes high.
When a detected temperature of the switching device rises, the thermal protection this device performs control in which, first, a high carrier frequency is switched to a low carrier frequency without limiting torque. Then, if the temperature still continues to rise, the device sets a torque limit value to be small.
Another related art is described in Japanese Patent Application Publication JP-A-7-322401.
Recently, environmentally friendly vehicles such as electric vehicles, hybrid vehicles, fuel cell vehicles and the like are receiving great attention. These types of vehicles are equipped with a motor that is driven by a direct-current power source and an inverter. The motor generates driving torque for the vehicle.
However, due to demand for reducing the cost and mounting space of the inverter which drives the motor, there is a trend toward smaller inverters with lower heat capacity. If such an inverter is utilized, a sharp rise in temperature is likely to occur due to heat concentration. This temperature rise is particularly apparent at high carrier frequencies where switching is frequently performed.
Note that a carrier frequency fc, which determines the switching frequency, is set based on the rotation speed of the motor and the required torque.
FIGS. 5A and 5B are conceptual diagrams that illustrate the carrier frequency.
FIG. 5A shows a case where the carrier frequency fc is 1.25 kHz. This carrier frequency is used as a basis for PWM controlling ON/OFF waveforms, thereby causing a current ICOIL to flow.
On the other hand, FIG. 5B shows a case where the frequency of the current ICOIL is higher than that in FIG. 5A. In this case, the carrier frequency fc has to be increased to 2.5 kHz to causing a current ICOIL to flow smoothly. The PWM control is performed at this carrier frequency to turn the switching device on and off.
By decreasing the carrier frequency instead of limiting the torque, the number of switching operations can be reduced. Thus, switching loss can be reduced, thereby suppressing the temperature rise by an amount that corresponds to the reduction in the switching loss. However, this does not necessarily allow the motor to rotate smoothly and thus vibration of the motor may increase.
FIG. 6 illustrates the carrier frequency and the temperature rise of the switching device.
FIG. 6 shows a case where the initial temperature is 65° C. In this case, when the carrier frequency fc is 1.25 kHz or fc is 2.5 kHz, the temperature of the switching device does not rise to 110° C., which is the temperature at which the switching device is damaged, even if the motor operation time is prolonged.
On the other hand, when the carrier frequency fc is 5 kHz, the switching loss becomes greater to the extent that the switching frequency is higher, and the switching loss generates heat. Therefore, if the initial temperature is 65° C. as in the cases where the carrier frequency fc is 1.25 kHz or fc is 2.5 kHz, the temperature may exceed 110° C. after a time t1. Accordingly, when the carrier frequency is high, the device is damanged unless countermeasures are taken.